Binders and mineral materials are among the main constituents of paints and of paper coating colours. The former, generally latex-based and in the form of aqueous suspensions or dispersions, provide the necessary adhesion to a support and cohesion between the elements forming goods to be produced, such as paint or paper but also such as agriculture fertilizer. The latter, commonly calcium carbonate, may provide improvements, for example, in paper and painting quality and agriculture properties, notably relative to its optical properties.
The concept of self-binding pigment particles is known to industry: it refers to distinct, solid particles, formed of mineral material and binder that are intimately bound to one another. The internal cohesion forces are such as to provide the self-binding pigment particles with excellent mechanical stability. Such particles may be directly implemented in a variety of applications.
Implementing self-binding pigment particles avoids the logistic difficulties of handling mineral materials and binders separately, and further avoids the unwanted physical and chemical interactions developed in comparable mixtures of minerals and binders.
Self-binding pigment particles are prepared by a process implementing at least one step of grinding mineral materials in the presence of binder, where grinding refers to an operation leading to a reduction in the particle size; the mineral materials in the self-binding pigment particle have a smaller diameter than the initial mineral material used to produce them. Such self-binding pigment particles are described in a number of documents, including WO 2006/008657, WO 2006/128814, and WO 2008/139292.
However, the processes for preparing self-binding mineral pigment particles known in the art are limited to the preparation or the co-grinding of suspensions having a low solids content. High solid content suspensions could only be processed if corresponding dispersants would be added as mentioned, e.g., in EP 1 747 252 which relates to a method for producing surface-modified inorganic fillers or pigments of a desired particle size. Said method is characterized in that filler or pigment slurries of inorganic fillers or pigments are milled to the desired particle size under the action of compressive and shear forces using polymer dispersions in combination with milling aids and/or dispersing agents.
The addition of dispersant, however, inter alia affects the adsorption of the binder to the particles during the co-grinding. The preparation of low solids content suspensions has the disadvantage that the obtained grinding product has to be up-concentrated before being shipped to other facilities for further processing. During the time and energy consuming up-concentrating step, very often an undesired loss of polymeric binder is observed, and, additionally, unwanted agglomerates are formed. Furthermore, the prior art processes often lead to a suspension having a high total organic content of the aqueous phase of the ground suspension.
In view of the foregoing, improving the self-binding pigment particle production process remains of interest to the skilled man. It would be especially desirable to provide a process for making self-binding mineral pigment particles which can be applied to mineral pigment suspensions having a high solids content, thus, avoiding an energy and time consuming up-concentrating step and, e.g., the formation of significant amounts of unwanted agglomerates.